Cellular radio systems are currently in widespread use throughout the world, providing telecommunications to mobile users. Companies that offer these mobile services, known as telecommunications service providers, generally operate within a given geographic region and frequency band. In order to meet capacity demand, while remaining within the allocated frequency band, the region is usually split into cells, at the center of each of which is a base station that handles communication with mobile stations. Communication channels, each occupying a portion of the allocated frequency band, are distributed among cells such that a given group of channels used in one cell is reused by certain other cells in the network. The distance between the reused cells is planned such that co-channel interference is maintained at a tolerable level.
The relative cost of a base station is usually quite significant, and telecommunications service providers are often interested in keeping infrastructure costs low by minimizing the number of base stations deployed to service a given region or, equivalently, increasing the size of individual cells. It is especially important to minimize infrastructure costs when low-capacity systems are envisaged, as the per capita cost of a base station can be prohibitive. Two realistic low-capacity scenarios in which it is imperative to optimize base station usage are residential or urban initial system roll-out and rural mobile telecommunications.
When a new cellular radio system is deployed in, say, a residential area, initial subscriber penetration can be quite low, and capital costs must be kept to a minimum until the telecommunications service provider has found enough subscribers to finance further investment in base stations. At the same time, providers must offer a decent coverage area so that a significant number of subscribers can be reached. Wide-area coverage with a minimum amount of base stations will therefore be of prime importance in an introductory low-capacity phase of the new network. Clearly, the success of a future high-capacity telecommunications network will ride on the ability to upgrade the initial low-capacity system without replacement of the base station in each cell.
In a rural mobile telecommunications scenario, mobile stations within a cell are usually spaced apart by considerable distances and a standard base station is at no risk of running out of available channels. However, conventional base stations lack the capacity to accommodate occasional new subscribers who might be located beyond present-day cell boundaries, and achieving this without incurring additional costs.
An excellent way to minimize infrastructure costs in both of the above scenarios is to improve the base station antenna technology so as to be able to provide wide-area coverage. A simple approach followed in the prior art consists of using an omnidirectional antenna and boosting the gain in order to increase its range. However, the gain of an omnidirectional antenna providing a significant improvement in range is on the order of 12 decibels (dB) or above, yielding a rather large and unsightly antenna whose installation will likely be objected to. Moreover, any such range increase is offset by a reduction in elevation beamwidth, a side-effect that is acceptable only in the rare case of a geographical area devoid of relief.
Another prior art method of increasing range consists of using several more compact directional antennas, and distributing them so as to cover a complete cell. Such a technique is known as cell sectoring and usually results in a cell being split into three sectors (a tri-sectored cell), although partitioning of a cell into two, four or six sectors is not uncommon practice. However, sectoring as performed in the prior art demands separate base station transceivers for each antenna and corresponding sector, leading to very high costs for a system whose capacity is not fully employed upon installation. Although it may be necessary to sector the cell in the long term to accommodate a growing subscriber base, the front-end expenditures associated with deploying multiple base station transceivers in an initial phase of the network are generally unacceptable.
Furthermore, transceivers at the base station of a given sector are dedicated to that sector, leading to significant levels of trunking inefficiency. In addition, the remainder of the telecommunications network, in particular the mobile switching center with which multiple base stations communicate, treats each sector as a separate cell. Considerable amounts of signalling and processing are therefore required at the base station and the mobile switching center in order to "hand off" a call as a mobile station moves from one sector to another within the same cell. The maximum capacity of a base station in this case is effectively truncated and new, supporting base stations may have to be added before long.
Clearly, no prior art method is capable of significantly increasing the range of a relatively low-capacity base station while keeping infrastructure costs to a minimum.